Apparatus for drying finely divided heat sensitive particles



July 7, 1970 BARRv v 3,518,772

APPARATUS FOR DRYING FINELY DIVIDED HEAT SENSITIVE PARTICLES Filed Sept. 30. 1968 i20-l40F United States Patent 3,518,772 APPARATUS FOR DRYING FINELY DIVIDED HEAT SENSITIVE PARTICLES Peter Joachim Barr, London, England, assignor to Ban & Murphy Ltd., London, England, a British corporation Filed Sept. 30, 1968, Ser. No. 763,697 Int. Cl. F26b 3/10 US. Cl. 34-10 5 Claims ABSTRACT OF THE DISCLOSURE A flash dryer arrangement is disclosed for handling finely divided materials such as starch and includes a main drying duct having a hot stream of gas flowing therethrough, the duct having a particle entry region defined by a venturi throat section serving to accelerate the hot stream of gas, a main section in which entrained particles are dried in part, while carried in the stream, and a ring section curving back to merge with the main section at a recycle point where the heavier moist particles separate from the lighter dried particles, the ring section finally leading to a cyclone separator.

The particle entry region has a perforate duct wall constituting a portion of the venturi section. A paddle wheel conveys the moist agglomerate material to the perforate wall and simultaneously rubs it through the perforate wall to provide finely divided particles that are immediately and directly contacted by the hot stream of gas to be distributed, entrained and carried in the stream in a state of fine division so that a maximum of surface area is exposed.

BACKGROUND OF THE INVENTION The present invention relates to a process and apparatus for drying finely divided heat sensitive particles whilst suspended in a stream of hot air or gas. It is well-known to flash dry finely divided particles in this way. Where the particles are of such a nature that they may be damaged if their temperature rises too high, certain problems arise in the flash drying process. In order to achieve economy in plant requirement it is conventional to utilize air at temperature much in excess of that at which the particles themselves might be burnt. It is therefore necessary to ensure that the particles have a high specific surface at the moment when they are exposed to the highest temperature zone of the hot air stream, so that their own surface temperature does not rise excessively, but remains comparatively cool owing to the rapid conversion of their liquid content into vapour at their surface.

It is wellknown that surface burning and partial gelatinisation are liable to occur in the production of dried powdered starch after dewatering of starch slurries in centrifuges or filters. The cake discharged from filters or centrifuges at a moisture content of 35 to 45% tends to agglomerate in conveying and feeding systems with the result that aggregates of starch are subjected to the hot air having the highest temperature in the flash drying systems. Since at this stage the ratio of surface to mass of the coarse aggregates is somewhat low there is a tendency for the aggregates to be overheated and damaged although moisture is flashed off from the aggregates.

For convenience, the apparatus of the present invention will hereinafter be described with reference to the processing of starch, but it will be readily understood that the apparatus finds equal applicability where other heatsensitive particles are to be dried.

It is an object of the present invention to provide a process and apparatus for flash drying moist particles in such a way that damage caused in a conventional systern is obviated. Basically this object is attained by re ducing the dewatered cake to a damp powder under conditions eliminating all possible damage to quality by either heat or impact and then cascading this damp powder into a current of drying air so rapidly and immediately that, in spite of the tendency of moist starch to form a-gglomerates, there is no time or opportunity for agglomerates to develop again and the damp powder is distributed, entrained and carried along by the drying air in a state of fine division so that a maximum of surface area is available for heat and mass transfer.

It has been found that a preferable way of reducing an agglomerate such as a soft filter cake to the state of a damp powder is to rub the filter cake through a screen. If the action applied to the filter cake is in a direction parallel to the plane of the screen so that pressure through the screen or anything resembling extrusion is avoided, the filter cake will break up into particles of a size much smaller than the perforations of the screen employed. In the preferred practice of the present invention it is proposed to incorporate the screen in the form of a perforated portion of the wall of the drying duct of a flash dryer, preferably in a duct section where the velocity of the drying air is especially increased, with the damp material brought to the ouside of the perforated Wall by means of a paddle wheel which fulfills the dual functions of bringing the damp cake to the perforated wall and rubbing the a'gglomerations through it. A further preferred feature of this invention is the provision of a cold air current which ensures that the paddle conveyor and the space outside the flash dryer is kept cool while assisting the product across the perforated wall. The volume of this cold air current would always be a very small fraction of the drying air employed in the flash dryer.

It is already well-known in the flash drying of airsuspended particles to separate drier particles from wetter particles by dividing the air stream in which the particles are carried, as the air stream passes around a sharp bend. In accordance with well-known principles the heavier wetter particles will be carried in the portion of the air stream which lies towards the outside of the bend. In the apparatus of the present invention this principle is used to retain a fraction of partially dried particles for further drying in the system with the result that in the average each particle recycles in the system with more heat interchange surface offered for better utilisation of the drying air so that a given final moisture content can be obtained at a lower air exit temperature than used in conventional systems with corresponding improvement of economy.

In order that the invention may be more fully comprehended reference is hereinafter made to the accompanying drawing, which shows in diagrammatic form an apparatus made in accordance with the invention.

In the apparatus shown the major part of the air passing through the drying duct is drawn in through a heater 1 in which it may be heated to a high temperature, for example 500 F. The drying duct in accordance with a preferred design, is divided into four parts, the section 2. being a venturi throat where the material to be dried is introduced through the perforated Wall 3. Further down stream the duct section 4, expands to normal carrying velocity for drying and transport to and beyond a recycle point 5 which is shown provided with adjustable deflectors 6. Further down stream the duct has the shape of a nearly closed ring 7 leading back to the recycle point 5 where the deflectors 6 are positioned to afford means for shaving oif a variable proportion of particles with some air for recirculation through the ring duct 7, with only the finest and driest fraction passing on through duct section 8 into the cyclone separator 9. Here the dried particles are recovered from the spent drying air. The air is drawn through the whole system, from the heater 1 to the a separator 9, by the fan 10 and the dried product is discharged through screw conveyor 11 and rotary valve 12.

Preferably the freshly filtered or centrifuged material is stored in the bin 13 and passed from there to the drying system by the controllable rate feeder 14 which delivers it to the paddle wheel 15. These paddles pass over the perforated wall 3 in such fashion that the partly agglomerated material coming from the bin 13 is rubbed through the screen and picked up by the drying air in the venturi throat where the drying air has its highest temperature and its highest velocity. A butterfly damper 16 controls admission of a small quantity of cooling atmospheric air into the casing 17 which houses the paddle wheel 15. This cooling air enters the main drying duct through the screen 3 but the quantity is kept so small that the temperature of the main air stream is not substantially reduced. The temperature of the air in instances of this type for drying starch is of the order of 300 to 500 F. at point of entry, and of the order of 120 to 140 F. at the exhaust 18 of the system with an intermediate value of approximately 200 F. at the recycle point 5.

The apparatus shown on the drawing is only one of many different embodiments for performing the process according to this invention and many alternatives will be obvious to those skilled in the art.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. Apparatus for flash drying finely divided material comprising a main drying duct, means for flowing a hot stream of gas through said duct, said main drying duct including duct wall means at a region therealong for accelerating the flow of the hot stream of gas at such region, means for introducing moist particles into said duct at said region and means for separating product particles proximate the outlet end of the main drying duct, said duct wall means being a perforate wall portion of said main duct and said means for introducing conveying aggregates of wet material to the outside of said perforate wall portion and simultaneously rubbing the aggregates through the perforate wall portion to enter directly into the hot stream of gas as finely divided moist particles.

2. Apparatus according to claim 1 wherein said means for introducing includes a paddle wheel for conveying and rubbing aggregates through said perforate wall portion, a casing housing said paddle wheel and means for controlling flow of a stream of cool airdraw'n through said casing and through said perforate wall portion by the action of said means for flowing the hot stream through said duct.

3. Apparatus according to claim 1 ,wherein said means for introducing includes a paddle wheel for conveying aggregates of wet material to the. outside of said perforate wall portion and for simultaneously rubbing the aggregates through the perforate wall portion to enter direcltly into the hot stream -of-gas as finely divided partic es. i

4. Apparatus according to claim 1 wherein said means for flowing includes a heater for gas entering the duct and a fan at the outlet end of the duct for drawing gas therethrough, said duct including a venturi throat section incorporating said duct wall means, said duct including a transport section downstream of said venturi throat section and a ring duct section leading around and back to a merger with an intermediate recycle point of the main duct section and then to an outlet that is in communication with said fan.

5. A method of flash drying material of a type having a tendency to agglomerate, said method comprising converting a moist agglomerate of the material by rubbing the same along and through a perforate entry zone to provide finely divided moist particles immediately thereadjacent, accelerating a hot stream of gas directly along the perforate entry zone to contact the moist particles directly upon formation and distribute, entrain and carry the same in said stream in a state of fine division so that a maximum of surface area is exposed, and changing the direction of the stream with the entrained particles to effect separation of the heavier moist particles therein from the lighter dried particles therein.

References Cited UNITED STATES PATENTS 2,235,748 3/1941 Hukill 3410 2,363,282 11/ 1944 Arnold.

2,538,833 1/1951 De Rycke.

3,354,555 11/1967 Barr.

FREDERICK L. MATTESON, JR., Primary Examiner R. A. DUA, Assistant Examiner 

